This invention relates to hydrolysis-resistant, thermoplastic molding compositions made of high-molecular weight, linear polybutylene terephthalate.
Polybutylene terephthalates are excellent materials for the manufacture of films, sheets, molded parts, etc. When used to manufacture molded parts by the injection-molding or extrusion methods, in particular, polybutylene terephthalate must have a viscosity number of &gt;120 cm.sup.3 /g.
Polybutylene terephthalates are customarily produced by transesterification or esterification and subsequent polycondensation of terephthalic acid or its polyester-forming derivatives and an alkanediol in the presence of catalysts. According to these processes, conducted in the melt, viscosities of the required magnitude, for example, for injection-molding and extrusion purposes, cannot be attained since, because of the thermal load, decomposition reactions occur as early as in the polycondensation stage so that the desired high viscosity cannot be reached. Another limitation on the viscosity of the polyesters in the melt condensation process results from the viscosity of the melt. The latter increases with progressive polycondensation, making it difficult or even impossible for the volatile, low-molecular weight reaction products to escape, thereby inhibiting the polycondensation progress. Furthermore, technical difficulties are encountered in discharging the high-viscosity melt from the reaction vessel (DOS No. 2,117,748).
If necessary, a solid-phase recondensation is carried out (British Pat. No. 1,066,162; U.S. Pat. No. 3,405,098). This is done by bringing the polyester, in granulated or powdered form, to temperatures ranging from 10.degree.-60.degree. C. below the melting point. In this temperature range, the polybutylene terephthalate is treated under an inert gas stream or under vacuum until the desired, high viscosity has been achieved. The recondensation can be effected either discontinuously, e.g., in a tumbler dryer (DOS No. 2,117,748), or continuously, for example in a fluidized-bed procedure (DOS No. 1,804,551) or in a screw-type reactor (DOS No. 2,162,618). A basic disadvantage of the solid-phase recondensation is derived from the long reaction period at elevated temperature; as a result, although a polybutylene terephthalate of the desired viscosity is obtained, it is already damaged to such an extent that it is greatly decomposed during melt processing. Molded components made of such a material are brittle and prone to breakage.
Therefore, it has been proposed to utilize polybutylene terephthalate in ground form to reduce the recondensation period; however, this process is not economical. Furthermore, there is a danger of caking of the powder (DOS's No. 2,152,245 and 2,117,748). German Pat. Nos. 3,033,468 and 3,033,469 propose a method wherein recondensation is conducted in the presence of alkanediols in two stages. This process has the drawback that it consumes a large amount of time.